A significant safety factor to be considered for systems that hold oil is that chambers inside the turbojet engines should be completely liquid-tight. Generally, the liquid-tight property of the chambers in turbojet engines is established by ensuring that a pressure difference is maintained between the outside of the chamber and the inside of the chamber at sealing limits of said chamber; the pressure difference should be such that the pressure inside the chamber is lower than the pressure outside the system by at least a value determined in advance.
As is shown in FIGS. 1 and 2, a jet pump system 102 is currently used to pressurise oil chambers in a turbojet engine 101, for example the LEAP-X®. Such a jet pump system, 102, also called an ejector or eductor, is a device that enables the pressure in a chamber to be improved when turbojet 101 is operating at low speed, while still ensuring that the pressure differential specifications at the chamber sealing limits are preserved. For this purpose, in the case of the LEAP-X® for example, ejector 102 draws in air at pressure P28 (P28 is the intake plan on the LEAP-X®, not generic), typically via a high pressure compressor 104 of turbojet 101 and mixes it with oil-free air 105 coming from chamber 103 at the centre vent tube. This supplementary air is injected by ejector 102 and creates an aspiration effect inside chamber 103, thus leading to a fall in pressure inside chamber 103 itself, and consequently a greater pressure differential at the sealing limits of chamber 103 that is being analysed. This device is necessary for the limits when turbojet 101 is running slowly when the pressure levels inside and outside the chamber are close to one another as well as close to the ambient pressure; it is therefore desirable to know the pressure differential that is needed.
Accordingly, in order to prevent oil leaks at low turbojet speeds, it is desirable to activate ejector 102 to ensure that a given pressure differential is maintained at the sealing limits of chamber 103. However, the extraction of air from inside high-pressure compressor 104 by means of ejector 102 is not neutral in terms of the performance of the turbojet, particularly during high-speed phases of the turbojet; this is why pressurisation of the chambers is most often maintained with regard to the exterior/interior pressure differential without the use of an ejector during high-speed phases.
It is therefore desirable to provide for controlled use of an ejector 102 to ensure that it is not used all the time. Accordingly, in the prior art use of control valve 106 for ejector 102 is provided, which is capable of switching from an on state, in which the pulsed air is directed into the centre vent tube, as the valve is in a completely open position, to a blocking state, in which no pulsed air is sent into the centre vent tube by the ejector because valve 106 is in a completely closed position.
The valve is designed for passive operation—it is called a passive valve—, that is to say the opening/closing movement of the valve, as shown in FIG. 3A, is controlled solely by a pressure differential that actuates it when the pressure differential reaches a trigger threshold DP0, this pressure differential being the difference in pressure between the P28 pressure drawn from high-pressure compressor 104 and the pressure surrounding the valve, or ambient pressure Pamb.
As is shown in FIG. 3B, the principle of operation of passive valve 106 includes a hysteresis 301 that offsets the operations of opening/closing the valve.
FIG. 4 represents a mapping 401 of different situations to which the passive valve may be exposed. Accordingly, in this figure, which has the form of an orthogonal coordinate system, the x-axis corresponds to a pressure differential DP maintained at the limits of passive valve 106, while the y-axis corresponds to an altitude at which valve 106 is located. The various situations—or operating points—are physically represented by triangles 402, which correspond to the case in which turbojet 101 is operating at low speed, and for which ejector 102 is in the on state, or by circles 403, which correspond to the case in which turbojet 101 is operating at high speed, and for which ejector 102 is in the blocking state.
As is shown in FIGS. 5A, 5B and 5C, a difficulty then arises in determining the activation threshold of one of the two configurations (ejector in on state or ejector in blocking state). In fact, if DP0 is defined as a constant regardless of the altitude under consideration, it is observed that:
as shown in FIG. 5A, high-speed situations 501 are complied with for a pressure differential level DP lower than threshold DP0, valve 106 then being open, the ejector thus being activated in such manner that engine performance may be impaired;
as shown in FIG. 5B, either low-speed situations 502 are maintained for a pressure differential DP level higher than threshold DP0, in which case valve 106 is closed, thus also deactivating ejector 102 with the associated risk of not satisfying the minimum pressure differential specifications for the purpose of liquid-tightness, and possibly allowing oil leaks to occur.
Moreover, as shown in FIG. 5C, there are situations 503 at low speed and high altitude for which it is possible that the ejector with its check valve fully open may create excessive aspiration within the chambers, thus leading to an excessively sharp loss of pressure in the oil chambers, so that this pressure falls to a level below a minimum pressure that is essential to ensure the proper functioning of the oil recovery pumps associated with the chamber.
In view of the above, it is desirable to provide a valve on the ejector with an activation system that:                enables the ejector to be activated only at low speed operating points in order to guarantee the minimum pressure differential at the sealing limits of the chamber, and also to avoid the risk of oil leaks occurring;        prevents air from being drawn from the high-pressure compressor at the high speed operating points in order to avoid impairing engine performance;        and beneficially reduces the power of the ejector in low speed phases and at high altitude in order not to interfere with the operation of the oil recovery pumps.        
It is clear that such a valve type is complex. A valve governed by the full authority digital engine control (FADEC) that satisfies these requirements exists, ensuring the proper function of the jet pump at the various operating points of the flight envelope of the LEAP-X®. However, this is a solution that requires a FADEC output and entails higher cost. Such a solution exceeds the definition limits of passive ejector valves because it relies on an electrical control.